all-or-none principle

The all-or-none principle says that a neuron's electrical pulse, called an action potential, either fires completely or does not fire at all — there is no in-between. Think of it like a camera flash: pressing the button halfway does nothing, but once you press hard enough, the flash goes off at full brightness every single time. A weak nudge to the cell produces no signal; a strong enough nudge produces a full one. There is no such thing as a half-sized spike.

This works because of a threshold, a kind of trigger point. As long as the push on the cell stays below threshold, nothing happens. The instant the push crosses threshold, a self-reinforcing chain reaction sweeps through the membrane and produces a spike of fixed size and shape, no matter how far past threshold the trigger was. So a neuron cannot say 'how strong' a signal is by making bigger spikes. Instead, it reports strength by firing faster — more spikes per second — and by how many neurons join in. This keeps signals crisp and reliable as they travel long distances, because every spike is regenerated at full strength and never fades along the way.